1. Field of the Invention
The present invention relates to the field of latching actuators.
2. Prior Art
Various types of latching actuators are well known in the prior art. By way of example, U.S. Pat. No. 3,683,239 discloses a self-latching solenoid actuator having a low power consumption and an internal switching arrangement whereby latching and unlatching may be accomplished by such means as a simple single pole double throw remote switch. In accordance with that disclosure the solenoid has a permanent magnet in the magnetic circuit thereof so that an actuating current in a first direction will actuate the solenoid and charge the permanent magnet, and a smaller current in the opposite direction will demagnetize the permanent magnet and allow a return spring to force the plunger to the fully extended position. A single pole double throw switch electrically coupled to the solenoid coil is disposed adjacent to the magnetic circuit and mechanically coupled to the solenoid plunger. The switch is coupled in circuit so as to be operative to turn off the actuating current and the unlatching current as the plunger approaches the latched and unlatched positions respectively, and to reconnect the solenoid coil in preparation for the next operating signal. Systems of this general form have been used commercially in sprinkler systems, such as those of U.S. Pat. No. 3,821,967 and 3,989,066.
One disadvantage of the foregoing system is the inclusion of the mechanical switch which introduces a mechanical failure mode as a result of the possible switch failure and/or improper switch positioning during the manufacturing process. The system also has the disadvantage that the actuator-mechanical switch combination is basically a three wire combination so that the turn on signal is provided through one line and set of switch contacts whereas the turn off signal is provided through a second line and a second set of switch contacts (the third line providing a return or ground). The three wire system is not of any particular disadvantage in sprinkler systems of the type hereinbefore referred to, though obviously, a three wire device is not compatible with control systems hereinbefore operating a conventional two lead nonlatching actuator, and is not directly interchangeable with such prior art nonlatching actuators.
Further, while control systems may be designed to control three wire actuators of the type hereinbefore described, the use of such three wire actuators, whether of this or of any other design, introduces additional required mechanism and/or circuitry and introduces failure modes which in most applications are not acceptable. In particular, conventional actuators actuate upon the application of a voltage thereto and release when the voltage is removed. Accordingly, a simple time clock or equivalent mechanism or circuit providing a simple switch closure between the actuator and a source of power for actuation and the opening of the same switch for release of the actuator will be all that is required. If one of the two leads is broken or the time clock switch is nonoperative, the actuator will remain in the released position. However in the three wire system of the general type described, one time clock switch must be provided to provide the turn on pulse and a second time clock switch must be provided to provide the turn off pulse. In addition to the additional mechanism and interconnections, the three wire system has the further disadvantage that a failure of the release time clock switch or the line carrying the release signal will still allow actuation of the actuator without a controllable subsequent release thereof, frequently a highly undesirable result because of the mechanical function of the actuator.
By way of a specific example, conventional actuators are used on the inlet water valve of household dishwashers. In a conventional system, when power is applied to the actuator (a two wire device), the actuator is actuated turning on the valve, and when power is removed therefrom, whether by way of intentional control or system failure, the valve will close. While it is true that the valves may stick and therefore fail to close, even though power is removed, the valve normally is only kept open for a minute or so at a time so that it has little time to freeze in the open position, i.e., if it turned on after sitting for a day or more, it should be capable of turning off shortly thereafter. In a three wire system of the general type described however, there are various types of failure modes such as the failure of the switch to provide the release pulse to the latching actuator and an open or poor contact on the third line. In any such failure, a water valve controlled by the actuator would remain on, leading to much more serious problems than a mere failure to actuate. Accordingly, while latching actuators have a number of very substantial advantages, in such applications they have not generally been used because of these problems.
One of the potential advantages of latching actuators in most applications is that the actuators may be considerably smaller than the corresponding nonlatching actuator because of their very low power consumption and energy dissipation in low duty cycle applications. In particular, nonlatching actuators must be held actuated during the entire actuated time period, normally with the number of ampere turns in the actuator coil approaching or equal to that which was required for actuation of the device when the air gap in the magnetic path was at its greatest. This results in considerable I.sup.2 R loss in the actuator coil, putting definite limitations on the minimum size coil and core that can be used. On the other hand, the current in a latching actuator coil only flows for a few milliseconds when the actuator is actuated, and a few more milliseconds when the actuator is released so that the instantaneous power dissipated in the coil may be much larger during the moments of actuation and release than could be tolerated if such current had to be sustained during the entire actuated time period. Thus, smaller cores and smaller coils may be used in a latching actuator used to replace a nonlatching actuator provided no substantial additional failure modes are introduced, particularly those failure modes which would be likely to leave the actuator in the actuated position.